1. Field of Invention
The present invention relates generally to a method of fabricating a ceramic substrate with a thermal conductive plug of a multi-chip package. More particularly, the present invention relates to a method of improving the fabricating process of the thermal conductive plug in the ceramic substrate in order to increase performance of the ceramic substrate of a multi-chip package.
2. Description of the Related Art
The demand of advanced electronic technology requires electronic products to be made lighter, thinner, faster, and smarter while simultaneously making them more friendly, powerful, reliable, robust, and less expensive. Thus, the trend for electronic packages is to develop highly-integrated packaging structures. Various techniques have been developed for having a large number of input/output points (I/O) in a device. The ceramic substrate is being widely utilized due to the desirable properties. Because the ceramic substrate has similar thermal expansion coefficient to the semiconductor device, thus a larger area of a chip can be used on the ceramic substrate and the detaching problem between the chip and the ceramic substrate can be prevented. Further, the ceramic substrate dose not absorb the moisture from the air, therefore, the ‘pop-corn’ effect and the peeling problem during the packaging can be effectively prevented.
Due to the complicated functions of the chip, a lot of heat dissipating problems are still required to be resolved. One of the methods of dissipating heat is to form a plurality of thermal conductive plugs under the die pad of the substrate. The heat generated from the chip will be conducted through the die pad and thermal conductive plugs to a ground plate located on the other side of the substrate. The ground plate is thermally conducted to other thermal conductive devices so that the heat generated from the chip can then be dissipated out to the external environment.
The conventional method of fabricating the thermal conductive plug inside the ceramic substrate is to perform the step of forming the thermal conductive plugs after cofiring the ceramic substrate. However, since the hardness of the ceramic substrate is very high, and the ceramic substrate is very brittle plus its measurement is very thick. Therefore, the only way to drill the thermal conductive holes in the ceramic substrate is by a laser method. A stencil printing method is carried out to fill the metal paste into the thermal conductive holes and followed by cofiring the metal paste so that thermal conductive plugs are formed in the thermal conductive holes.
From the above-mentioned fabrication, the formation of the thermal conductive holes on the ceramic substrate has to be by laser. However, in order to penetrate through the high hardness of thick ceramic substrate, a lot of energy is required. The high power of a laser machine is very expensive. On the other hand, the size of the metal conductive plug will be reduced after cofiring the metal paste in the thermal conductive holes. After cofiring, the size of the thermal conductive holes remains unchanged and the size of the thermal conductive plugs are shrunken, therefore, voids will be formed in the thermal conductive holes. The thermal conductivity will be inefficient and those voids will lead to produce rents in the ceramic substrate. The worst is that the thermal conductive plugs will even fall out from the holes. As a result, specific materials for the metal paste have to be chosen to use so that the volume will not shrink but rather expand during the cofiring process. However, the fabrication is then limited to the selection of the materials of the metal paste.